The venous system includes a multitude of one-way bicuspid valves that permit substantially unidirectional blood to flow toward the heart. These valves are particularly important in the lower extremities to prevent the pooling of blood. When the leaflets of the bicuspid valves fail to close properly, the valve is considered “incompetent” as it permits leakage of retrograde flow resulting in the abatement of flow towards the heart.
This potentially serious condition is known as “chronic venous insufficiency.” Symptoms can progress from unsightly “spider” or varicose veins to skin discoloration and painful skin ulcerations. The etiology of venous insufficiency is multifactorial, including a past history of thrombotic events, chronic venous stasis, and venous hypertension. Current treatments for venous insufficiency include elevation of the feet and compression stockings. While these can relieve symptoms, the underlying disease remains untreated. Surgical techniques are also employed in which native valves can be bypassed or replaced with autologous sections of veins having functioning valves.
Recently, various implantable medical devices and minimally invasive methods for implantation of these devices have been developed to deliver these medical devices within the lumen of a body vessel. These devices are advantageously inserted intravascularly, for example from an implantation catheter. For example, implantable medical devices can function as a replacement venous valve or aortic valve, or restore native venous or aortic valve function by bringing incompetent valve leaflets into closer proximity. Such devices can comprise an expandable frame configured for implantation in the lumen of a body vessel, such as a vein. Implantable valve devices can further comprise features that provide a valve function, such as leaflets.
However, post-implantation thrombosis and platelet deposition on surfaces of endovascular prosthetic valves may occlude the conduit defined by the endovascular prosthesis or compromise the functionality of an implanted valve by limiting the motion or responsiveness of moveable portions of the device such as valve leaflets. For example, stagnation of blood around implanted prosthetic valves may cause stiffening and thickening of valve leaflets, reducing the leaflets' functionality and possibly eventually occluding the body lumen.
In a natural valve, the leaflets are typically located within a sinus or enlargement in the vein. For example, the portion of the aorta that serves as the anchorage of the valve leaflets, or the aortic root, consists of three aortic sinuses. It is believed that the pockets formed between the leaflets and the walls of the sinus create vortices of flowing blood that help flush the pocket and prevent blood from stagnating and causing thrombosis around the valve leaflets, which can interfere with the function of the valve. It is thought that the stagnating blood prevents oxygen from reaching the endothelium covering the valve cusps, leading to hypoxia of the tissues which may explain increased thrombus formation typical in that location. Expandable-frame valve prostheses typically are of a generally cylindrical shape and lack an artificial sinus or pocket space that is sufficient for simulating these natural blood flow patterns.